Descriptions of heat-sensitive recording paper that produces a recorded image by thermal reaction between an electron donating colorless dye precursor (hereinafter referred to as a color former) and an electron accepting substance (hereinafter referred to as color developer) are set forth in Japanese Patent Publication Nos. 14039/1970 and 4160/1968. The use of heat-sensitive recording paper has expanded to terminal printers connected to electronic computers and instrumentation equipment of various types, as well as to facsimile recording apparatus.
When heat-sensitive recording paper is brought into contact with chemicals or oil, the recorded image may either lose its color or develop fog. These problems are serious and must be solved in order that heat-sensitive recording paper may gain wide commercial acceptance.
In order to meet this requirement, a protective layer is conventionally formed on the heat-sensitive recording layer, as is described in Japanese Patent Application (OPI) Nos. 30347/1973, 31958/1973, 19840/1978, 14751/1979, 53545/1979, 111837/1979, 128349/1979, 126193/1981, 139993/1981, 10530/1982, 29491/1982, 105392/1982, 107884/1982, 53484/1983, and 193189/1983 (The term "OPI" as used herein referes to a "published unexamined Japanese Patent Application"). The commercial value of the heat-sensitive recording paper having such protective layer can be increased by providing the back side of the support with a backcoat layer containing a water-soluble polymeric binder and an inorganic pigment because this backcoat layer not only improves the running properties of the recording paper but also prevents the occurrence of sheet curling and imparts chemical resistance to the back side of the support.
However, because of the water-soluble polymeric binder it contains, the conventional backcoat layer has low water resistance and swells in a moist atmosphere, sticking to the protective layer on an underlying sheet of heat-sensitive recording paper (this sticking phenomenon is also referred to as blocking). Therefore, the commercial value thereof is inferior. In order to prevent the occurrence of blocking, which leads to unsalable products, the use of an alkali salt of isobutylene-maleic anhydride copolymer has been described Japanese Patent Application (OPI) No. 9091/1984, but the water resistance of the backcoat layer containing such alkali salt is still unsatisfactory.